[lwip-devel] [task #7865] implement non-blocking SEND operation (socket)

[Jonathan Larmour]

Follow-up Comment #2, task #7865 (project lwip):

Jim Pettinato wrote:
> Do you really need to implement another queue, or could you just
> have it return an error eg ERR_WOULD_BLOCK if there is no longer
> room on the existing TCP send queue, and let the task retry
> later? 

Well that's a tricky thing I should probably have highlighted. A full send
queue is not the only thing that can cause tcp_enqueue to return ERR_MEM. It
can also happen if you run out of MEMP_TCP_SEGs, pbufs of the needed sort,
having a resulting tcp queue which ends up too long once this write is added,
and more. Have a look.

Superficially you might think that that's ok if you just try the tcp_write
and look for ERR_MEM, but how does an application then know when more
resources become available? ie. an app waiting in lwip_select() has to be
blocked (so tcp_write returning ERR_MEM would imply :
      API_EVENT(conn, NETCONN_EVT_SENDMINUS, len);
So what would be calling:
      API_EVENT(conn, NETCONN_EVT_SENDPLUS, len);
in that case since the lack of resource may be unconnected to the send data
enqueued for this tcp pcb.



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